Technical Field
The embodiments described herein relate to portable electrical energy storage devices, such as those used in electric powered devices such as vehicles, and consumer electronics and thermal runaway mitigation systems for such portable electrical energy storage devices.
Description of the Related Art
Batteries such as lithium-ion batteries are known for packing more energy into smaller, lighter units. Lithium-ion batteries have found wide application in powering portable electronic devices such as cell phones, tablets, laptops, power tools and other high-current equipment. The low weight and high energy density also makes lithium-ion batteries attractive for use in hybrid vehicles and fully electric-powered vehicles.
A potential shortcoming of lithium-ion batteries is their electrolyte solutions. Unlike other types of batteries, in which the electrolytes consist of aqueous solutions of acid or base, the electrolyte in lithium-ion cells typically consists of lithium salts in organic solvents such as ethylene carbonate and ethyl methyl carbonate (which can be flammable).
Under normal operation, charging a lithium-ion battery causes lithium ions in the electrolyte solution to migrate from the cathode through a thin porous polymer separator and insert themselves in the anode. Charge balancing electrons also move to the anode but travel through an external circuit in the charger. Upon discharge, the reverse process occurs, and electrons flow through the device being powered.
In very rare circumstances, internal or external short-circuiting of a lithium-ion battery can occur. For example, the electric-powered device containing the lithium-ion battery can undergo a severe impact or shock resulting in a breach in the battery, which could result in a short circuit. Due to the thin nature of the polymer separator, micrometer-sized metal particles generated during cutting, pressing, grinding, or other battery manufacturing steps can be present or find their way into the battery cell. These small metal particles can accumulate and eventually form a short circuit between the anode and the cathode. Such short circuits are to be avoided because they can result in temperatures at which the cathode may react with and decompose the electrolyte solution, generating heat and reactive gases such as hydrocarbons. Typically, at normal operating temperatures, lithium-ion batteries are very stable; however, above a certain temperature, lithium-ion battery stability becomes less predictable, and at an elevated temperature, chemical reactions within the battery case will produce gases resulting in an increase in the internal pressure within the battery case. These gases can react further with the cathode, liberating more heat and producing temperatures within or adjacent to the battery that can ignite the electrolyte in the presence of oxygen. When the electrolyte burns, small amounts of oxygen are produced, which may help fuel the combustion. At some point, build-up of pressure within the battery case results in the battery case rupturing. The escaping gas may ignite and combust. Some battery manufacturers design their cells so, in the unlikely event a cell ruptures and ignites, gases that support combustion exit the cell in predetermined locations and directions. For example, battery cells in the shape of conventional AAA or AA cells may be designed to vent from the terminal ends located at each end of the cell.
In applications where only a single lithium-ion battery is utilized, failure of a battery and the potential for combustion creates an undesirable situation. The severity of this situation is increased when a plurality of lithium-ion batteries are deployed in the form of a battery bank or module. The combustion occurring when one lithium-ion battery fails may produce local temperatures above the temperature at which other lithium-ion batteries are normally stable, causing these other batteries to fail, rupture, and vent gases which then ignite and combust. Thus, it is possible for the rupture of a single cell in a bank of lithium-ion cells to cause other cells in the bank to rupture and discharge gases which ignite and burn. Fortunately, lithium-ion batteries have proven to be very safe, and the failure and consequent rupture of a lithium-ion battery is a very rare event. Nonetheless, efforts have been made to reduce the risk of rupture and ignition of gases exiting a ruptured lithium-ion battery. For example, development of materials used for cathodes has produced lithium-based cathode materials that tolerate heat better than cathodes made from the widely used lithium cobalt oxide. While these more recently developed materials may be more heat tolerant, this benefit comes at a price. For example, lithium manganese oxide cathodes have a lower charge capacity than lithium cobalt oxide and still decompose at high temperatures. Lithium iron phosphate cathodes stand up especially well to thermal abuse; however, their operating voltage and energy density on a volume basis are lower than those of lithium cobalt oxide cathodes.
Other efforts have focused on the polymer separator and its design. For example, it has been proposed to utilize a polymer separator that sandwiches a layer of polyethylene between two layers of polypropylene in an effort to provide a degree of protection against mild overheating. As the temperature of the cell begins to approach that at which the stability of the cell becomes unpredictable, the polyethylene melts and plugs the pores in the polypropylene. When the pores of a polypropylene are plugged by the polyethylene, lithium diffusion is blocked, effectively shutting the cell down before it has a chance to ignite. Other efforts have focused on utilizing polymer separators having melting points higher than polypropylene. For example, separators made from polyimides and separators made from high molecular weight polyethylene and an embedded ceramic layer have been proposed to form a robust higher melting point polymer separator. Formulating and utilizing less flammable electrolytes and nonvolatile, nonflammable ionic liquids, fluoroethers, and other highly fluorinated solvents as battery electrolytes have also been investigated. Researchers have developed lithium-ion batteries that contain no liquids at all. These solid-state batteries contain inorganic lithium-ion conductors, which are inherently nonflammable and are thus very stable, safe, and exhibit long cycle life and shelf life. However, the manufacture of these solid-state batteries requires costly, labor-intensive vacuum deposition methods.
Despite these efforts, there continues to be a need for a portable electrical energy storage device that effectively manages the risk of electrical energy storage cell failure and combustion of gases produced as a result of such failure in multi-cell deployments, as well as propagation of failure inducing thermal energy to battery cells adjacent a failed cell, and the hazard to the user in the event of such a rare event.